<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>判断按下的是不是a，如果是弹a，如果不是就弹出按下的键</title>
</head>
<body>
  <input type="text">
  <script>
    var ipt = document.querySelector('input')
    ipt.onkeyup = function (e) {
      // String.fromCharCode：将数值转换为字母
      var n = String.fromCharCode(e.keyCode)
      console.log(e.keyCode, n.toLowerCase())
    }
  </script>
</body>
</html>